This invention relates to electronic tags adapted to be attached to articles and more specifically to a tamper-sensing device in a tag which senses whether the tag has been removed from one article and applied to another.
In general, an electronic tag is a device that can be affixed to a particular article to respond with digital information identifying the article when the tag is externally interrogated. Such tags are commonly used in inventory control in both commercial and governmental applications. In many instances, the tagged articles are either hidden from view or are not readily observable by the inspector. In such cases, communication channels, such as telephone lines, fiber optic cables or radio must be used to connect the tagged article with a remote inspector.
A proposed use of electronic tags is in the automotive field wherein automobiles and trucks may be tagged for automated toll collection. In such case, a vehicle passing through a toll collection station can have its tag interrogated by radio frequency signals. The tag would respond with a digital identification, such as the manufacturer's vehicle number. The toll would then be billed to the owner of the vehicle. In like manner, the owner of a fleet of vehicles could interrogate each vehicle by cellular telephone to get information about the vehicle.
For electronic tags to serve their purpose, they must remain attached to the article they are designed to identify. In many instances a person might find it very advantageous to remove the tag from one article and transfer it to a second, or surrogate, article. In such case, if the tag on the surrogate article still gave the original identification signal, the interrogator would not realize a switch had been made.
As a consequence, it is desirable that a tag have a tamper sensor that is integrated with the article to provide a unique and unvarying digital tamper-sensing signal, usually in digital form, indicating that the tag is still affixed to that article. To prevent undetected removal and fixing of the tag to a surrogate article, the tamper sensor should be designed so that if the tag is still capable of generating a tamper-sensing signal, such signal will be different than before. Thus, if the tag when interrogated responds with no tamper-sensing signal at all, or a different tamper-sensing signal, then the interrogator will know the tag has been removed from the original article.
In instances where it could be highly advantageous to remove a tag and affix it to a surrogate article, a sophisticated adversary with knowledge of the particular digital tamper-sensing signal emitted from the tag when affixed to the original article might be able to remove the tag, affix it to a surrogate and adjust the electronics of the tag so that it would again give the original tamper-sensing signal. To prevent this, the tag should be designed so that the digital value of the particular tamper-sensing signal can be known only by an authorized interrogator.
In addition, the tamper sensor should have the advantages of being small in size, easy to install, passive (i.e. not requiring internal batteries), low in maintenance, relatively inexpensive, secure against physical, chemical, x-ray or electronic attacks, with low (preferably no) false alarms, and with the ability to function as a seal for doors or the like.